The invention relates to a method for producing biphenols by oxidative coupling of dialkylphenols. More particularly it is directed to a method which proceeds in two stages using a copper amine complex which is catalytically effective in each stage. Still more particularly it relates to a novel copper amine complex with high catalytic activity for the oxidative coupling of substituted phenols under mild conditions which has dual (two stage) activity and can be readily recycled and reused.
The invention is a process for producing tetraalkylbiphenols and, more particularly, 2,2xe2x80x2,6,6xe2x80x2-tetraalkyl-4,4xe2x80x2-biphenols by the stepwise oxidative coupling of 2,4-dialkylphenols or 2,6-dialkylphenols in the presence of a copper amine complex. The reaction of the 2,6-dialkylphenones is preferred. The first step of this reaction is the synthesis of 3,3xe2x80x2,5,5xe2x80x2-tetraalkyl-4,4xe2x80x2-diphenoquinones using oxygen as an oxidizer-to couple the 2,6-dialkylphenols. The second step is the synthesis of 2,2xe2x80x2,6,6xe2x80x2-tetraalkyl-4,4xe2x80x2-biphenols using 3,3xe2x80x2,5,5xe2x80x2-tetraalkyl-4,4xe2x80x2-diphenoquinones as an oxidizer to couple 2,6-dialkylphenols. (Both steps usually employ an excess of the phenol.) In accordance with the invention, the same catalyst can be used in both steps. In addition, the reaction proceeds under extremely mild conditions used in the preferred embodiments of the invention and this results in highly selective syntheses in both steps. Thus in the first step of the preferred process only 3,3xe2x80x2,5,5xe2x80x2-tetraalkyl-4,4xe2x80x2-diphenoquinones are produced, and in the second step only 2,2xe2x80x2,6,6xe2x80x2-tetraalkyl-4,4xe2x80x2-biphenols are produced. Low temperature prohibits dealkylation of biphenols, which results in impurities such as polyphenols.
The reaction of the 2,4-dialkylphenones proceeds directly to the 2,2xe2x80x2-diphenol and does not proceed via the diphenoquinone.
The extremely high activity of the catalytic system used in the preferred embodiment of the invention enables the use of only a very modest amount (i.e. about 0.05 to 1% based on dialkylphenol) of catalyst. In addition, the oxidation proceeds well at modest concentrations of oxygen; thus headspace air pressures as low as 1 to 100 psi are useful and high pressures for oxygenation are not required. The catalyst is readily removed from the product stream and recovered for reuse. Typically, essentially no impurities are produced. The system described below also can permit the efficient recovery of any unreacted 2,6-dialkylphenol starting materials, permitting their reuse without subsequent additional purification.
With specific reference to the reaction of 2,6-dialkylphenol, it has been discovered that 2,2xe2x80x2,6,6xe2x80x2-tetraalkyl-4,4xe2x80x2-biphenols can be efficiently produced in a two-step process by using a copper-amino salt complex with dual catalytic activity and two oxidizing agents, first oxygen, then the corresponding 3,3xe2x80x2,5 xe2x80x2-tetraalkyl-4,4xe2x80x2-diphenoquinone intermediate, to couple 2,6-dialkylphenols. The process utilizes a solvent, which significantly reduces the temperature at which the coupling occurs, permitting high selectivity to be achieved. In addition, the solvent permits the recovery of excess 2,6-dialkylphenol and catalyst for reuse without subsequent additional purification.
The catalyst has improved catalytic activity compared to a similar system described in U.S. Pat. No. 4,851,589. Unlike the catalyst described in that patent, the catalyst preferably used in the present invention, does not require an activation period, the use of acidic phenols, and can be removed from the products while retaining its full activity and reused. In addition, the catalyst can be prepared, recovered and stored in closed containers for over one year while retaining full catalytic activity, eliminating the need to prepare fresh catalyst for each reaction.
In accordance with another embodiment of the invention, tetraalkylbiphenols are dealkylated to produce a 4,4xe2x80x2-biphenol and its analogs. In a preferred embodiment, this reaction is conducted in the presence of a low boiling solvent to enhance and facilitate the removal of isobutylene and prevent the formation of polyisobutylene.
In accordance with the preferred embodiments of the invention, the catalyst is prepared by reacting a copper halide (e.g., cuprous bromide, cupric chloride, cupric bromide, and preferably cuprous chloride (CuCl)), with a tetraalkylalkylenediamine. The preferred diamine is N,N,Nxe2x80x2,Nxe2x80x2-tetramethyletylenediamine (TMEDA) but other diamines that have been used in the preparation of copper amine complexes could also be used such as alkylamines, dialkylamines, arylamines, pyridines etc. In the preferred embodiment of the invention the copper-amino complex is prepared under aerobic conditions in the polar organic solvents such as acetone, tetrahydrofuran (THF), alkyl esters such as ethyl acetate, butyl acetate etc., other ketones such as methyl ethyl ketone (MEK), etc., and ethers such as dimethyl ether, methyl-t-butyl ether (MTBE), etc. These solvents are selected because the catalyst is essentially insoluble and precipitates as a dark brown solid. The catalyst clearly differs in its properties from the catalyst described in the U.S. Pat. No. 4,851,589. It is prepared in a different solvent resulting in precipitation and the dark brown solid that is recovered cannot be completely dissolved in methanol or ethanol. In addition, it can be readily removed from the products of the reaction while retaining its activity, permitting it to be re-used in subsequent reactions. In the presence of oxygen it converts 2,6-dialkylphenols to 3,3xe2x80x2,5,5xe2x80x2-tetraalkyl-4,4xe2x80x2-diphenoquinones. Under the conditions described herein, the diphenoquinones are typically produced exclusively and not as a mixture of 3,3xe2x80x2,5,5xe2x80x2-tetraalkyl-4,4xe2x80x2-diphenoquinones and 2,2xe2x80x2,6,6xe2x80x2-tetraalkyl-4,4xe2x80x2-biphenols which typifies other processes. The same catalyst catalyzes the oxidative coupling of the dialkylphenol and the reduction of the diphenoquinones in the second stage of the reaction.
The invention is not limited to the use of the preferred copper-amino complex. Known copper-amino complexes can also be adapted for use in the two stage dual catalytic reaction of 2,6-dialkylphenols as described herein.
Process Conditions
In a typical process in accordance with the present invention, the 2,6-dialkylphenol to be coupled is dissolved in methanol. While methanol is clearly the preferred solvent, those skilled in the art will recognize that other solvents, in particular other alcohols such as ethanol, isopropanol or butanol can be used. A solvent is selected which dissolves the dialkylphenol and the catalyst and water. It is also important to be able to remove water from this solvent, if it is to be reused in the next cycle. In addition to alcohols ketones may also be useful. The concentration of the 2,6-dialkylphenol is typically about 1 to 5 molar.
An excess of the phenol is used for the reaction. As little as 0.05% by weight of catalyst based on phenol can be effective and is added to the solution of the phenol. The preferred catalyst is completely soluble in the methanolic solution of the 2,6-dialkylphenol. Oxygen or air is bubbled directly into reaction mixture, or is delivered to the headspace of the reactor at a modest pressure (i.e., about 5 to 20 psi air). The amount of oxygen is readily determined based on the stoichiometry of the reaction. The temperature of the reaction can be easily and preferably controlled in the range of about 30 to 50xc2x0 C. Higher temperatures can be used particularly with solvents having a higher boiling point than methanol but in the interest of maintaining the selectivity of the reaction lower temperatures are preferred. Methanol serves several of important purposes: it keeps the viscosity of the reaction mixture low while the 3,3xe2x80x2,5,5xe2x80x2-tetraalkyl-4,4xe2x80x2-diphenoquinones are being produced, affording ease of agitation and efficient mixing; it freely dissolves water, the byproduct of reaction, keeping the catalyst active; methanol is also used to separate the catalyst and excess 2,6-dialkylphenol from the final product for reuse in subsequent reactions.
Typically after approximately 35 to 40% of initial 2,6-dialkylphenols have been converted to the corresponding 3,3xe2x80x2,5,5xe2x80x2-tetraalkyl-4,4xe2x80x2-diphenoquinones, oxygen addition may be stopped and reaction carried out in the absence of oxygen. At this time the methanol solvent is removed by distillation for re-use, then the water produced as a by-product of the reaction is removed by distillation at a higher temperature and disposed of. For the second stage reaction, the temperature needs to be raised just enough to keep reaction mixture in a liquid form, typically about 130 to 160xc2x0 C. The presence of the catalyst permits the intermediate 3,3xe2x80x2,5,5xe2x80x2-tetraalkyl-4,4xe2x80x2-diphenoquinones to serve as oxidizing agents, for the subsequent coupling of the 2,6-dialkylphenols. Higher temperatures may be used if higher concentrations of diphenoquinone are used but it is not desirable to exceed temperatures of 200xc2x0 C. because catalyst will thermally decompose and dealkylation may occur. After the 3,3xe2x80x2,5,5xe2x80x2-tetraalkyl-4,4xe2x80x2-diphenoquinone is consumed, the reaction mixture generally contains about 70 to 80% 2,2xe2x80x2,6,6xe2x80x2-tetraalkyl-4,4xe2x80x2-biphenols and 20 to 30% 2,6-dialkylphenols. The reaction mixture is then cooled to approximately 60xc2x0 C. and the methanol that was removed by distillation is added back to the reaction mixture. In a period of several minutes the 2,2xe2x80x2,6,6xe2x80x2-tetraalkyl-4,4xe2x80x2-biphenols precipitate completely and can be removed by filtration or centrifugation, while the 2,6-dialkylphenols and the active catalyst remain in the filtrate or centrate. Following the addition of fresh 2,6-dialkylphenol and optional addition of fresh catalyst, these mother-liquors can be re-used to start another cycle of producing of 2,2xe2x80x2,6,6xe2x80x2-tetraalkyl-4,4xe2x80x2-biphenols.
The 2,6-dialkylphenols which can be reacted in accordance with the invention are represented by formula: 
where R1 is an alkyl group having 1 to 6 carbons, and R2 is alkyl group having 3 to 6 carbons. In order to prevent coupling through the oxygen atom and production of a mixture of Cxe2x80x94O coupled oligomers, one of the alkyl groups should have at least three carbon atoms to provide steric hindrance with respect to the phenolic oxygen atom and thereby prevent reaction. While the principal focus of the invention is on the preparation of tetraalkylbiphenols, R1 and R2 could also be C1 to C6 alkoxy moeities.
The reaction of the 2,6-dialkylphenol is shown in the following equations:
Step 1: 
Step 2: 
The catalytic system described in this patent can also be employed in the one-step oxidative coupling of 2,4-dialkylphenols: 
where R1 and R2 are alkyl groups having 3 to 6 carbons. In the case of R1 or R2 having less then 3 carbons, Cxe2x80x94O coupling will occur and the process will produce a mixture of Cxe2x80x94O coupled oligomers. This reaction does not proceed through the quinone, but otherwise this reaction proceeds under conditions which are substantially the same as those disclosed above for the first stage reaction of the 2,6-dialkylphenol. The reaction is typically conducted using the temperatures, solvents and concentrations described herein for the first stage reaction.
The catalytic system described in this patent can also be employed in the one-step oxidation of 2,2xe2x80x2,6,6xe2x80x2-tetraalkyl-4,4xe2x80x2-biphenols. 
where R1xe2x80x94R2 are alkyl group having 1 to 6 carbons. The reaction is typically conducted using the temperatures, solvents and concentrations described herein for the first stage reaction. Catalyst may be suspended in the reaction mixture.
While the invention has been described with respect to coupling like dialkylphenols, those skilled in the art will recognize that the invention can also be used to couple dialkylphenols that are different. The latter reaction would merely yield a mixture of products, which could be separated in a conventional manner.
Dealkylation
The products of the reactions described herein can be used directly as antioxidants, UV-absorbers, and specialty monomers. 4,4xe2x80x2-Biphenol can be produced from 2,2xe2x80x2,6,6xe2x80x2-tetra-t-butyl-4,4xe2x80x2-biphenol through dealkylation.
Many of the methods of dealkylation that are most commonly known suffer from several major problems, notably the large if not complete loss of isobutylene to form polyisobutylene, an impurity which is very difficult to remove from the final product. Other problems with prior methods of dealkylation include large losses due to alkylation of solvents, long reaction times unless the reaction is conducted at high temperature, and low purity of the 4,4xe2x80x2-biphenol product. These problems can be attributed to the fact that alkylation and dealkylation as well as the polymerization of isobutylene are promoted by the same catalyst in conventional processes.
In accordance with another embodiment of the invention a method of dealkylation is provided which permits essentially complete dealkylation to be achieved at comparatively low temperatures while permitting recovery of more than 90% of isobutylene with high purity, e.g., exceeding 99.5%. The 4,4xe2x80x2-biphenol produced typically has a purity exceeding 99.5% without recourse to further purification by crystallization or distillation.
The method of dealkylation comprises the use of a strong acid and a solvent mixture in which a minimum of one component has a boiling point about 20 to 50xc2x0 C. lower then the temperature of the reaction mixture. The dealkylation reaction is usually carried out at a temperature of about 130 to 170xc2x0 C. Examples of the low boiling solvent are hydrocarbons with 7-9 carbons, halogenated hydrocarbons with boiling point about 80 to 130xc2x0 C. Preferably about 5 to 30% by volume of the solvent mixture is a low boiling solvent. The use of this solvent mixture keeps the reaction mixture saturated with vapors of this low-boiling component and very efficiently removes isobutylene from the reaction mixture before it polymerizes or alkylates the solvent or the biphenols in the reaction mixture. The use of such solvent system also efficiently removes any moisture, which may be present in the reaction mixture. Removing moisture dramatically reduces the reaction time by a factor of two to ten, important for the economics of large-scale production. Examples of acids that can be used in the dealkylation include sulfonic acids such as methanesulfonic acid, which is preferred, sulfuric acid, toluenesulfonic acid, aluminum phenoxides etc.